The present invention generally relates to engines. The invention particularly relates to recovery of energy from exhaust gases produced by an internal combustion engine.
Today's society largely relies on internal combustion engines for transportation, commerce and power generation, particular examples of which include earth moving equipment, tractors, aircrafts, and ships. An example of an internal combustion (IC) engine is the Otto cycle engine used in most automotive vehicles these days. An IC engine operating on the principles of the Otto cycle comprises four strokes namely, intake, compression, power, and exhaust, which occur within what is commonly termed a combustion chamber or cylinder. During the intake stroke, a finite volume of air, determined by what is commonly referred to as the cylinder swept volume, is drawn into the combustion chamber along with a quantity of fuel for compression during the compression cycle and subsequent combustion of the fuel during the power cycle. The mass of this finite volume of air is dependent on the ambient temperature and pressure from which the air is drawn. Because air density increases as temperature decreases, a larger mass of air can be drawn into the combustion chamber at colder ambient temperatures, as compared to higher ambient temperatures. The ratio between the mass air flow into the engine and the cylinder swept volume is known as the volumetric efficiency (ηv) of the engine. The typical volumetric efficiency of a normal IC engine at open throttle is between about seventy-five and ninety percent.
The internal combustion engine powers millions of passenger cars and other vehicles. These vehicles consume fuel to extract energy which accounts for the consumption of millions of barrels of crude oil per day. This high fuel demand leads manufacturers to increase the efficiency of engines and make them more fuel efficient. By utilizing a turbocharger, the size of engines can be reduced with minimal reduction in power output.
A turbocharger is a device which forces a higher volume of air into the combustion chamber of the engine during the intake stroke, increasing the oxygen content in the chamber for combustion and the volumetric efficiency of the engine. This has a direct correlation on the amount of torque and power produced. A turbocharger generally includes a compressor driven by a turbine, which in turn is driven by the flow of exhaust gases from the engine, specifically by exhaust gas recirculation (EGR). A supercharger is a similar device, but instead of EGR powering the turbine, power is drawn from the crankshaft of the engine via mechanical linkages.
Current commercially available turbochargers generally suffer from various limitations. Power lag or turbocharger lag is the time required to change power output in response to a throttle change (that is, the time between when the driver presses the accelerator pedal creating a torque request and when the turbocharger actually produces power), noticed as a hesitation or slowed throttle response when accelerating as compared to a naturally aspirated engine. This is due to the time needed for the exhaust system and turbocharger to generate the required boost. Inertia, friction, and compressor load are the primary contributors to power lag.
Because turbochargers are physically driven by the flow of exhaust gases, their performance is influenced as a result of exhaust gases exiting internal combustion engines in pulses, rather than a constant flow. Turbochargers inherently promote the generation of exhaust gas back pressure due to the turbine vane intrusion. This back pressure may hinder the in-cylinder pressure of cylinders within the engine, which have open exhaust valves during the exhaust stroke, and can affect the exhaust stroke pumping work and pressure. The back pressure may also increase the residual burnt mass fraction in the cylinders which can lead to advanced ignition and retardation of the 50% burn crank angle, causing reduction in combustion efficiency and increased emissions. However, turbochargers are very susceptible to the exhaust gas pressure and for low pressure conditions, turbochargers may generate less than optimal in-cylinder pressures.
Often, not all of the exhaust gases produced by the engine is used to drive the turbine, and a portion of exhaust gases is bypassed through a waste gate into the tailpipe. This excess bypass does zero work and can be a significant source of energy loss. Capturing this energy could reduce waste and increase the efficiency of the engine as more work is done by the same chemical bond energy released by the fuel.
In addition to the above limitations, turbochargers are also generally loud while running at higher rpm, and depend on oil for lubrication that is not available until after the engine has started. Turbochargers also tend to be expensive due to their highly detailed geometry.
In view of the above, it can be appreciated that it would be desirable if systems and methods were available for promoting the efficiency and performance of internal combustion engines that induct compressed air into their combustion chambers while eliminating or reducing the problems, shortcomings or disadvantages associated with turbochargers.